On Mon, 18 Jun 2012, Jeffrey Lee Hellrung, Jr. wrote:

On Mon, Jun 18, 2012 at 12:34 PM, Jeremiah Willcock <jewillco@osl.iu.edu> wrote:
     On Mon, 18 Jun 2012, Jeffrey Lee Hellrung, Jr. wrote:

           On Mon, Jun 18, 2012 at 12:25 PM, Jeremiah Willcock <jewillco@osl.iu.edu> wrote:
                On Mon, 18 Jun 2012, Jeffrey Lee Hellrung, Jr. wrote:

                      On Mon, Jun 18, 2012 at 10:35 AM, Jeremiah Willcock <jewillco@osl.iu.edu> wrote:
                           Boost.Preprocessor has a macro BOOST_PP_INTERCEPT to eat a numeric value that it is token-concatenated onto.  Is
           there a
                      similar macro that just
                           returns the value?  I.e., some BOOST_PP_EMPTY_FOR_CONCAT such that BOOST_PP_EMPTY_FOR_CONCAT ## 3 turns into 3?  As
           with
                      BOOST_PP_INTERCEPT, it
                           only needs to work for small integer values.  Is there some other technique I can use for this?  I am using it to
           generate
                      std::get<>
                           invocations in BOOST_PP_ENUM_BINARY_PARAMS.  Thank you for your help.


                      I don't know any such facility in Boost.PP, but it's pretty trivial to generate:

                      #define X0 0
                      #define X1 1
                      #define X2 2
                      // etc.

                      ...which makes me think I might be overlooking something in Boost.PP :/

                      Somewhat related: what do your ENUM_BINARY_PARAMS invocations look like? I know I've gotten away with cat'ing template
           parameter
                      delimiters ("<" and ">")
                      against preprocessor integer tokens.


                I'm not using this code anymore, but it was basically:

                BOOST_PP_ENUM_BINARY_PARAMS(nparams, std::get<ZZZ, >(tup) BOOST_PP_INTERCEPT)

                where ZZZ is the identity-type macro discussed in the email.  Even using something that I would expect to work like + as ZZZ
           breaks since
                apparently signs aren't allowed at the beginnings of pp-tokens.  I'm using GCC 4.7, and it seems to be strict about
           concatenations.

            
           So, just to confirm,

           BOOST_PP_ENUM_BINARY_PARAMS( nparams, std::get<, >(tup) BOOST_PP_INTERCEPT )

           (i.e., with ZZZ being an empty token) does not work with gcc 4.7?


     Unless "empty token" means something other than putting nothing there, it expands to what I want but produces errors:

     foo.cpp:3:1: error: pasting "<" and "0" does not give a valid preprocessing token
     foo.cpp:3:1: error: pasting "<" and "1" does not give a valid preprocessing token
     foo.cpp:3:1: error: pasting "<" and "2" does not give a valid preprocessing token
     foo.cpp:3:1: error: pasting "<" and "3" does not give a valid preprocessing token
     foo.cpp:3:1: error: pasting "<" and "4" does not give a valid preprocessing token
      std::get< 0 >(tup) , std::get< 1 >(tup) , std::get< 2 >(tup) , std::get< 3 >(tup) , std::get< 4 >(tup)

     When trying to expand:

     BOOST_PP_ENUM_BINARY_PARAMS(5, std::get<, >(tup) BOOST_PP_INTERCEPT)


Sounds like I might need to change my preprocessor programming habits :(

How about using a leading 0?

BOOST_PP_ENUM_BINARY_PARAMS( nparams, std::get< 0, >(tup) BOOST_PP_INTERCEPT )

I was using numbers large enough that marking them as octal (or even hex) would break, so adding 0 or 0x won't work.  I guess you could use:

BOOST_PP_ENUM_BINARY_PARAMS(nparams, std::get<1000, %100>(tup) BOOST_PP_INTERCEPT)

though.